The present invention relates to turbo-charged gasoline engines of the type comprising:
at least one cylinder and an intake valve and an exhaust valve associated with the cylinder,
an intake conduit and an exhaust conduit connected to the cylinder,
a compressor interposed in the intake conduit, for supercharging the cylinder,
a turbine interposed in the exhaust conduit and mechanically coupled to the compressor, to exploit the exhaust gases from the engine in order to drive the supercharger compressor,
sensor means of the engine's operating conditions,
control means of the cylinder compression ratio, adapted to vary the opening time of the intake valve, and
an electronic unit that controls the aforementioned control means of the compression ratio, according to the output signals of said sensor means, in order to decrease the compression ratio as the engine speed increases, in order to prevent or reduce the need to deviate part of the exhaust gases in a by-pass conduit connected in parallel to the turbine.
An engine of the type specified above is, for example, described and shown in U.S. Pat. No. 5,255,637. According to such known solution, it is envisaged to reduce or completely eliminate the intervention of the so called “waste-gate” valve, which controls the flow rate of the exhaust gases through the aforementioned by-pass conduit. Such a valve is normally used in turbo-charged engines in order to prevent too high supercharger pressures at high engine speeds. In such conditions, part of the gases combusted is discharged in the by-pass conduit, instead of being directed through the turbine. In the engine shown in U.S. Pat. No. 5,255,637, the intervention of the “waste-gate” valve is reduced or eliminated thanks to the predisposition of means for reducing the compression ratio of the engine, through a variation of the closing moment of the intake valve (before or after the bottom dead centre) when a sensor associated with the combustion chamber detects a trend towards detonation.